Something, such as a glove or other pledge, thrown down as a challenge to combat (now usually figurative).

1819, Walter Scott, Ivanhoe:

“But it is enough that I challenge the trial by combat — there lies my gage.” She took her embroidered glove from her hand, and flung it down before the Grand Master with an air of mingled simplicity and dignity…

1988, James McPherson, Battle Cry for Freedom, Oxford 2003, page 166:

The gage was down for a duel that would split the Democratic party and ensure the election of a Republican president in 1860.

[I]t seemed to create a sort of material link between the Princess and himself, and at the end of three months it almost appeared to him, not that the exquisite book was an intended present from his own hand, but that it had been placed in that hand by the most remarkable woman in Europe.... [T]he superior piece of work he had done after seeing her last, in the immediate heat of his emotion, turned into a kind of proof and gage, as if a ghost, in vanishing from sight, had left a palpable relic.

1747, Berry, Helen, quoting Anonymous, The Life and Character of Moll King, late mistress of King's Coffee House in Covent Garden[1], quoted in "Rethinking Politeness in Eighteenth-Century England", Transactions of the Royal Historical Society, published 2001, page 75, volume 11, series 6:

Harry. To pay, Moll, for I must hike.Moll. Did you call me, Master?Harry. Ay, to pay, in a Whiff.Moll. Let me see. There's a Grunter's Gig, is a Si-Buxom; two Cat's Heads, a Win; a Double Gage of Rum Slobber, is Thrums; and a Quartern of Max, is three Megs: — That makes a Traveller all but a Meg.Harry. Here, take your Traveller, and tip the Meg to the Kinchin.